1. Field of the Invention
The present invention concerns a new low-pressure casting process intended in particular to enable the same installation to be used for casting alloys under highly diverse conditions, in particular alloys of aluminum and magnesium.
2. Description of the Prior Art
The low-pressure casting process has been known since the beginning of this century. Using this technique:
a metal or otherwise mold is filled from the bottom with a liquid alloy contianed in a hermetically sealed furnace; the alloy may rise into the mold through an injection tube;
filling of the mold is achieved with the aid of a fluid at a pressure of a few decibars;
after the mold is filled, a deadhead overpressure is maintained while the part solidifies;
the non-solidified alloy in the injection passages in the bottom of the mold is recovered as soon as the part has solidified and after the injection pressure is removed.
The molds employed may be permanent (in which case they are fabricated in cast iron or steel or graphite) or non-permanent in which case they are destroyed after casting to release the part. Such non-permanent molds are fabricated from chemical sand, ceramic or plaster.
The alloys used are generally those of aluminum, applications in the automobile industry being of particular importance, and also those of magnesium and copper (brass, bronze). Significant developments are currently in progress in connection with cast iron and steel.
In the case of aluminum alloys the gas used to pressurize the furnace is generally air, although it may be nitrogen to avoid oxidation of the surface. The manufacture of the aluminum alloy and operation in air may introduce hydrogen into the alloy because of the moisture content of the fusion product, the ambient air and the air fed into the furnace. It is therefore necessary to degas the metal at the end of manufacture and during casting.
This operation may be done by chemical agents (special fluxes, chlorine, nitrogen, etc) or by applying a vacuum of a few millibars to the surface. In this case it may be beneficial to generate the vacuum in the low-pressure casting furnace.
In order to avoid interrupting casting for this vacuum degassing to take place it is possible to use two furnaces, one being used for casting and the other for degassing and taking over from the first when it is empty. To this end the two furnaces are movable on rails.
Mobile furnaces are known in the art and there also exist low-pressure furnaces for casting aluminum designed to withstand a vacuum. The pressure-tight outer jacket is also vacuum-tight. One disadvantage of this approach is that the refractory and insulative block supporting the furnace elements and thermally insulating the furnace are exposed to the vacuum. This block is porous and contains a significant quantity of gas and even of moisture, and it has to be maintained continuously at a temperature in excess of 80.degree./100.degree. C. after a long drying time, several days before it is put into service for the first time. Also, these materials are often fibrous or powdery and are sucked in by the vacuum pumps.
These disadvantages, associated with the fact that sealing is provided by the outer jacket, are compensated by the fact that this jacket runs cool (50.degree. to 80.degree. C.) as it is protected from the elements by the insulating material and is therefore not subjected to thermal fatigue stresses during each pressurization cycle. These stresses would otherwise require significant reinforcing of the structure. Likewise, applying a vacuum to a jacket at high temperature would produce deformations in the absence of appropriate reinforcements. Because of this an open plate is all that is placed before the elements, in order to protect them mechanically.
Low-pressure casting machines with provision for vacuum casting of aluminum are therefore of this type, with a single sealed jacket on the outside of the furnace, as shown in FIG. 1 in the appended drawings.
FIG. 1 shows a sealed outer jacket 1, a fixed lid 2 attached to this jacket, a mobile lid 3, elements 4, refractory members 5 supporting the elements, insulators 6, a protective plate 7 open at the bottom, a crucible 8, a metal 9, an injection tube 10, a pressurizing air inlet 11, a depressurizing air intake 12, a vacuum air offtake 13, an outlet 14 for evacuating the metal should the crucible break, lid insulation 15, seals 16 and 17, a connecting nozzle 18, a fixed plate 19 of the casting machine and a mold 20.
In the case of magnesium alloys, however, this device cannot be used since the gas employed to inject the magnesium must not bring about any oxidation of the metal, the currently known solution being to use sulphur hexafluoride (SF6) diluted to 0.5 to 1% in air or carbon dioxide.
This gas is a fluoride and decomposes the refractory and insulating material, however, as these are generally based on silica compounds (silica-alumina in particular). It is therefore necessary to protect the refractory materials from the SF6 and a low-pressure casting machine for magnesium has to have a sealed inner jacket that can serve as the crucible to contain the metal.
FIG. 2 is a diagram of a machine of this kind, showing an outside jacket 1' of the furnace which no longer as any sealing function, a crucible 2' containing the magnesium, a lid 3', elements 4', refractory materials 5', insulating materials 6', molten magnesium alloy 7', an injection tube 8', a special injector nozzle 9' with provision for protective gas, an inlet 10' for feeding the gas based in diluted SF6 into the connecting nozzle, an inlet 11' for feeding SF6 into the furnace to move the metal, a decompression gas outlet 12', a seal 13', a fixed plate 14' of the machine and a mold 15'.
In a furnace of this kind the crucible 2' contains the metal at approximately 750.degree. C. and its outer wall is heated by the elements. During each cycle it is subjected to thermal fatigue stresses and is internally pressurized, generally to a pressure of 0.6 to 1 bar. These stresses combine with the risks of corrosion which can only be limited in the case of dilute SF6 by using high-chromium steel. This leads to very thick crucibles (up to 20 mm) which are costly, difficult to handle and somewhat dangerous to use.
Note, however, that magnesium alloys are not subjected to vacuum degassing as they vaporize at significant pressures, often in the order of 60 millibars.
These factors are such that current low-pressure casting machines for aluminum and for magnesium are different, while in both cases the solutions adopted for depressurizing the aluminum and pressurizing using dilute SF6 are unsatisfactory.
Foundries which cast parts of the aerospace industries use both aluminum alloys and magnesium alloys and these parts, which are generally large, thin and of high metallurgical quality, are typical applications of low-pressure casting.
An object of the present invention is therefore to define the structure of a new type low-pressure casting machine capable of supporting:
vacuum in the order of 1 millibar; PA1 pressures up to 2 bars; PA1 various non-corrosive gases such as air or nitrogen and corrosive gases such as SF6;
and all this without being exposed to thermal fatigue stresses during pressurization or to creep forces due to suction when the vacuum is applied.